Hi John,
You're correct, Parc des Expositions is the first stop after CDG 1 station at CDG Airport.
RER Billet Ile-de-France tickets can be purchased from the ticket window within the
CDG 1 train station (or
CDG 2 train station). You don't need to specifically ask for a Billet Ile-de-France ticket, the ticket seller will know which type of ticket you need. I wouldn't recommend trying foreign credit cards; they most likely will not work with neither the automated ticket vending machines, nor at the ticket windows. So have a little bit of Euro cash handy. There is a HSBC bank machine at the Terminal 2 train station, top floor.
Best regards,
Ben